A common method to indirectly measure unsteady surface shear uses a “hot” wire or “hot” film on the surface and is well known in the art. Surface shear is a tangential force exerted on a surface (wall) caused by flow moving over the surface. This method determines surface shear by measuring cooling of the wire or film at the surface and comparing it to known cooling velocity values. This technique is the only known method to measure high-frequency surface shear fluctuations. In addition to only indirectly measuring surface shear, the “hot” wire or “hot” film method does not measure directional changes of the flow. To remedy the indirect nature and directional ambiguity of the prior art, surface-shear measurements via shear balances (a shear sensor that includes a strain gauge or other methods known in the art) mounted flush with the surface were developed. These devices would directionally deflect under the action of shear. Although capable of detecting direction and amount of surface shear, the bulkiness of their floating elements made them only useful for time-averaged measurements over a large area, rather than at a single point (never less than 1.0×1.0 mm2).
The emergence of micro electro mechanical systems (MEMS) technology generated considerable hope in constructing micron-sized floating elements, or shuttles, with a sensing area less than 0.5×0.5 mm2 and a bandwidth of tens and even hundreds of kHz (i.e., much greater temporal resolution).
The excitement over MEMS floating elements resulted in a number of attempts to construct high precision directional shear flow sensors. Although some success was achieved in constructing and testing the MEMS floating elements, one problem was soon realized. The extremely small area on which the surface shear acts can only produce Angstrom-size deflections. Thus, in order to maximize the deflection, the shuttle support could only be a few microns wide. This rendered these sensors fragile and, for all practical purposes, only useable by their makers under highly controlled conditions. Additionally, the minute deflections of the floating element within the shuttle did not seem to produce sufficient signal-to-noise ratio, particularly when using capacitive pickups known in the art for detecting the deflection.
Other attempts to remove directional ambiguity from hot wire measurements included the use of “pulsed-wire” anemometers previously used for velocity measurements in separated flows. This technology applied to measuring surface shear stress was later developed using a sensor having a central heating wire surrounded by upstream and downstream cold wires. A central wire, typically oriented at 90 degrees with respect to the sensor wires, is heated periodically. Fluid velocity is measured from the time of heating the central wire until a change in temperature is detected by one of the cold wires (time of flight). Flow direction (forward or reverse) is determined by which cold wire changes temperature.
Unfortunately, there are several difficulties and limitations using this pulsed anemometry technology. First, to avoid thermal diffusion effects, the sensing volume size is typically no less than one to two millimeters. This limits sensor spatial resolution. This sensor separation limits the frequency response to tens, or a few hundred Hz at best (i.e., low temporal resolution). Second, in flows with large velocity gradients, such as near surfaces (walls), the measurements must be corrected using constants. Finally, pulsed hot wires require elaborate and careful calibration. Again, this limits their application since they are impractical for applications involving array measurements.
A different variation on pulsed anemometry also known in the art uses three parallel wires to measure the fluid velocity in a one-dimensional pulsating flow such as in a pipe. In this approach, a central wire is operated as a conventional constant-temperature sensor and used to measure the magnitude of the velocity. Flow direction is indirectly determined by incorporating the two outside wires in opposite legs of a Wheatstone bridge to form a thermal tuft, known in the art, on the wall under a re-attachment zone of a backward facing step. Although this method overcomes some of the disadvantages of the time of flight technique, the frequency bandwidth remains limited to tens or a few hundred Hz due to separation of the thermal tuft sensors and their thermal inertia.
Thus, there remains a need to develop a flow measurement device that has high spatial (less than 1.0×1.0 mm2) and temporal resolution (greater than 10s to 100s kHz) to measure fluid flow properties in unsteady and direction-reversing fluid flows.